1. Field of the Invention
The present invention relates to a signal generating device, and more particularly, to a signal generating device for selectively generating a plurality of different signals.
2. Description of the Related Art
Various types of devices are known that produce a digital signal by reading a gray code pattern portion provided on a code plate by means of a reading member. For example, a brush provides positional information on an object.
FIG. 5 is a schematic view of a conventional signal generating device which includes a brush and a code plate having a gray code pattern portion consisting of a plurality of patterns.
In FIG. 5, patterns P0, P1 and PC of a conductive substance, each of which generates a bit signal, are formed on a code plate 1 to form a gray code pattern portion. Reading of the gray code pattern portion is conducted to obtain bit signals by moving a brush 2 within a range from a position A to a position E in a state where it is in contact with the gray code pattern portion. The code plate 1 may be made movable, whereas the brush is fixed.
In the aforementioned structure, reading of the gray code pattern portion is changed over at points B, C and D. That is, the entire gray code pattern portion is divided into four regions I to IV.
The obtained bit signals are transferred to another printed circuit board (not shown) through a connecting terminal portion 1a. Thereafter, they may be input to operation means, such as a microcomputer, and used for various controls.
The above-mentioned conventional device has only one combination of the change-over points (hereinafter referred to as dividing positions). Application of such a signal generating device to, for example, detection of a focal length or a distance to an object in a single-lens reflex camera causes the following problems.
First, each type of single-lens reflex camera requires divided portions different from those required by the other types to detect the focal length or the distance to an object. Thus, it is difficult to employ a common code plate in various types of cameras, and one type of camera having a code plate with a gray code pattern portion different from those of other camera types must be provided. However, provision of a different code plate for each camera type increases production cost, particularly with regard to the type of camera which is manufactured only in small numbers at one time.
Secondly, a common code plate may be prepared by increasing the number of bits or increasing the divided regions. However, this increases the number of input pins of a microcomputer which inputs signals from the signal generating device and processes them, and requires a microcomputer having greater complexity. Hence, a large size microcomputer is necessary, which is disadvantageous in terms of production cost and installation space.